Thermal management system for fuel cell vehicle having multiple fuel-cell stacks

ABSTRACT

A vehicle includes first and second fuel-cell stacks, a first coolant circuit having conduit arranged to circulate coolant through the first fuel-cell stack, a second coolant circuit having conduit arranged to circulate coolant through the second fuel-cell stack, a heater in fluid communication with at least the first coolant circuit, and an isolation valve assembly configured to proportion a flow of coolant between the first and second coolant circuits. The isolation valve assembly includes valving. The valving has an isolation position in which the first and second circuits are isolated. The valving also has at least one mixing position in which the first and second circuits are in fluid communication.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 16/660,164filed Oct. 22, 2019, now pending, the disclosure of which is herebyincorporated in its entirety by reference herein.

TECHNICAL FILED

This disclosure relates to vehicle having multiple fuel-cell stacks andmore specifically to thermal management systems for thermally regulatingthe fuel-cell stacks.

BACKGROUND

The hydrogen fuel cell, and in particular the proton exchange membranefuel cell (PEMFC), is one potential power source for automobiles andstationary applications. The reaction in a PEMFC involves hydrogenmolecules splitting into hydrogen ions and electrons at the anode, whileprotons re-combine with oxygen and electrons to form water and releaseheat at the cathode. Typically, a proton exchange membrane is used as aproton conductor in a PEMFC. A catalyst layer containing, for example,platinum and/or a platinum alloy is used to catalyze the electrodereactions. A gas diffusion layer, which may include a microporous layerand a gas diffusion backing layer, is used to transport reactant gasesand electrons as well as remove product water and heat.

Excessively cold or hot fuel cell temperatures may affect the membraneand other materials in the stack. Fuel cell systems typically includethermal management systems to control the temperature of the fuel-cellstack within a desired temperature range.

SUMMARY

According to one embodiment, a vehicle includes first and secondfuel-cell stacks, a first coolant circuit having conduit arranged tocirculate coolant through the first fuel-cell stack, a second coolantcircuit having conduit arranged to circulate coolant through the secondfuel-cell stack, a heater in fluid communication with at least the firstcoolant circuit, and an isolation valve assembly configured toproportion a flow of coolant between the first and second coolantcircuits. The isolation valve assembly includes a first inlet portconnected to the conduit of the first circuit, a first outlet portconnected to the conduit of the first circuit, a second inlet portconnected to the conduit of the second circuit, a second outlet portconnected to the conduit of the second circuit, and valving. The valvinghas an isolation position in which the first inlet port and the secondoutlet port are not in fluid communication and the second inlet port andthe first outlet port are not in fluid communication so that the firstand second circuits are isolated. The valving also has at least onemixing position in which the first inlet port and the second outlet portare in fluid communication and the second inlet port and the firstoutlet port are in fluid communication so that the first and secondcircuits are in fluid communication.

According to another embodiment, a vehicle includes first and secondfuel-cell stacks and a thermal management system. The thermal managementsystem includes a first coolant circuit including conduit arranged tocirculate coolant through the first fuel-cell stack, a first pump, and afirst radiator; a second coolant circuit including conduit arranged tocirculate coolant through the second fuel-cell stack, a second pump, anda second radiator; and a third coolant circuit including conduitarranged to circulate coolant through a heater, a third pump, and aheater core. A first valve arrangement is configured to selectivelyconnect the third circuit to the first circuit and configured toselectively connect the third circuit to the second circuit. A secondvalve arrangement is configured to proportion a flow of coolant betweenthe first and second coolant circuits.

According to yet another embodiment, a vehicle includes first and secondfuel-cell stacks, a first coolant circuit having conduit arranged tocirculate coolant through the first fuel-cell stack, a second coolantcircuit having conduit arranged to circulate coolant through the secondfuel-cell stack, and a heater in fluid communication with at least thefirst coolant circuit. A valve arrangement is configured to proportion aflow of coolant between the first and second coolant circuits. The valvearrangement includes an isolation position in which the first and secondcircuits are not in fluid communication and at least one mixing positionin which the first and second circuits are in fluid communication. acontroller is programmed to, in response to coolant of the first circuitbeing less than a first threshold, command the isolation valve assemblyto the isolation position and command the heater ON, and, in response tocoolant of the first circuit exceeding a second threshold, command theisolation valve assembly to the at least one mixing position and commandthe heater OFF.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a fuel-cell vehicle.

FIG. 2 is a schematic diagram of a thermal management system.

FIG. 3 is a schematic diagram of a valve arrangement according to oneembodiment.

FIG. 4 is a schematic diagram of a valve arrangement according toanother embodiment.

FIG. 5 is a schematic diagram of the thermal management system in apreconditioning mode for warming the first fuel-cell stack prior tostarting.

FIG. 6 is a schematic diagram of the thermal management system in awarm-up mode of the first fuel-cell stack.

FIG. 7 is a schematic diagram of the thermal management system in afuel-cell-to-fuel-cell warmup mode.

FIG. 8 is a schematic diagram of the thermal management system in acabin-and-battery heating mode.

FIG. 9 is a schematic diagram of the thermal management system in a modewhere the first stack is ON and the second stack is OFF.

FIG. 10 is a control diagram for the thermal management system.

FIG. 11 is a flowchart of an algorithm for controlling the thermalmanagement system during a cold start.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention. As those of ordinary skill in the art will understand,various features illustrated and described with reference to any one ofthe figures can be combined with features illustrated in one or moreother figures to produce embodiments that are not explicitly illustratedor described. The combinations of features illustrated providerepresentative embodiments for typical applications. Variouscombinations and modifications of the features consistent with theteachings of this disclosure, however, could be desired for particularapplications or implementations.

A PEMFC includes a proton exchange membrane (PEM). The anode and thecathode typically include finely divided catalytic particles, usuallyplatinum, supported on carbon particles and mixed with an ionomer. Thecatalytic mixture is deposited on opposing sides of the membrane. Thecombination of the anode-catalytic mixture, the cathode-catalyticmixture, and the PEM form a coated catalyst membrane electrode (CCM). Inorder to facilitate the transport of reactant gases to and remove theexcessive water and heat from the catalytic mixture, a gas diffusionlayer (GDL), which may include a microporous layer and acarbon-fiber-based gas diffusion backing layer, may be applied on eitherside of the CCM to form a membrane electrode assembly (MEA). GDLs alsoprovide mechanical support for the soft goods including the PEM andcatalytic mixtures.

MEAs are sandwiched between bipolar plates to form unit cells. Thebipolar plates typically include an anode side and a cathode side. Anodefuel flow channels are provided on the anode side of the bipolar platesthat allow the anode gas to flow to the anode side of the MEA. Cathodeoxidant flow channels are provided on the cathode side of the bipolarplates that allow the cathode gas to flow to the cathode side of theMEA. Coolant channels may be disposed between the anode and cathodesides of the bipolar plates to thermally regulate the fuel cell.

Several unit cells are typically combined in a fuel-cell stack togenerate the desired power. For example, the stack may includetwo-hundred or more unit cells arranged in series. The fuel-cell stackreceives a cathode reacting gas, typically a flow of air forced throughthe stack by a compressor. Not all the oxygen is consumed by the stackand some of the air is output as a cathode exhaust gas that may includewater as a stack byproduct. The fuel-cell stack also receives an anodehydrogen reacting gas that flows into the anode side of the stack.

Referring to FIG. 1, a vehicle 10 includes a first fuel-cell stack 20and a second fuel-cell stack 22 for providing electrical power to atleast one electric machine 12. The vehicle 10 may also include atraction battery 14 electrically connected to the fuel cells 20, 22 andthe electric machine 12. The electric machine 12 is connected to thedriven wheels 16 via a drivetrain 18. During operation of the vehicle10, hydrogen fuel and air are fed into the fuel cell 20 creatingelectrical power. The electric machine 12 receives the electrical poweras an input, and outputs torque for driving the wheels 16 to propel thevehicle 10.

The fuel-cell stacks 20, 22 generate heat during operation and includean associated thermal management system for thermally regulating thetemperature of the stacks. In addition to cooling the stacks 20, 22, thethermal management system is also configured to heat the stacks. It isdifficult to start a fuel cell in very cold ambient conditions such asbelow −25 degrees Celsius (C) for example. (Starting a fuel cell whenthe temperate is below a threshold may be referred to as cold start.) Toimprove cold start, the fuel-cell stacks 20, 22 may be preheated (alsoknown as preconditioning) by circulating heated coolant through one ormore of the stacks 20, 22. Once the stack reaches a thresholdtemperature, such as −25 degrees, the fuel cell is started. This ofcourse is just an example temperature and the threshold temperature willvary based on fuel cell design and other factors. The preheating canended once the fuel cell is started as heat from the chemical reactionswill self-heat the fuel-cell stack to a desired operational temperature,at which point active cooling may be required. The preheating may beperformed by an electric heater such as a positive temperaturecoefficient (PTC) heater that converts electricity, typically providedby the battery 14, into heat.

The following figures and related text describe an example thermalmanagement system of the vehicle 10. Referring to FIG. 2, a thermalmanagement system 30 thermally regulates at least the first and secondfuel-cell stacks 20, 22 and may also be responsible for thermallyregulating traction battery 14 and supplying heat to the passengercabin. The thermal management system 30 uses liquid coolant, such asethylene glycol mixed with deionized water, as the working fluid. Thethermal management system 30 may include a first coolant circuit 32associated with the first stack 20 and a second coolant circuit 34associated with the second stack 22.

The first circuit 32 may include a pump 36, a radiator 38 with anassociated fan 40, a valve 42 (e.g., an electric three-way valve or waxthermostat), temperature sensors 46, 48, and conduit 44 that is acollection of multiple lines, hoses, pipes, tubes, etc., arranged toform a closed loop. In the illustrated embodiment, the valve 42 is anelectric three-way valve and a temperature sensor 43 is provided todetermine actuation of the valve 42. While not shown, the first circuit32 may include a pressure sensor and other flow control devices such ascheck valves. The conduit 44 carries the coolant through the associatedcomponents of the first coolant circuit 32. The term “conduit” may referto the collection of all conduit of the circuit 32 or a specificsegment(s) of the conduit depending upon context. The first circuit 32is configured to heat or cool the stack 20 depending upon the relativetemperatures of the coolant and the fuel-cell stack 20. When the systemis active, the pump 36 circulates coolant into an inlet 50 of the stack20, the coolant then travels through the internal cooling system of thestack 20 exchanging thermal energy, and exits via an outlet 52. Fromthere, the conduit 44 conveys the coolant to the valve 42. The valve 42operates to either route the coolant to the radiator 38 if the coolantis too hot or bypasses the radiator via the bypass line 54. The valve 42may be configured to proportion coolant between the radiator 38 and thebypass line 54. The fan 40 is arranged to increase air flow through theradiator 38 if needed. The fan 40 may be electronically controlled. Thetemperature sensor 46 may be located just upstream of the fuel-cellstack 20 to measure the temperature of the coolant at the inlet 50. Thetemperature sensor 48 may be located just downstream of the fuel cell 20to measure temperature of the coolant at the outlet 52.

The second circuit 34 may include a pump 60, a radiator 62 with anassociated fan 64, a valve 66 (e.g., an electric three-way valve or awax thermostat), temperature sensors 68, 70, 71, and conduit 72 that isa collection of multiple lines, hoses, pipes, tubes, etc., arranged toform a closed loop. The conduit carries the coolant through theassociated components of the first coolant circuit 34. The secondcircuit 34 is configured to heat or cool the stack 22 depending upon therelative temperatures of the coolant in the fuel-cell stack. When thesystem is active, the pump 60 circulates coolant into an inlet 74 of thestack 22, the coolant then travels through the internal cooling systemof the fuel cell exchanging thermal energy, and exits via an outlet 76.From there, the conduit 72 conveys the coolant to the valve 66. Thevalve 66 operates to either route the coolant to the radiator 62 if thecoolant is too hot or bypasses the radiator via the bypass line 78. Thevalve 66 may be configured to proportion coolant between the radiator 62and the bypass line 78. The fan 64 is arranged to increase air flowthrough the radiator 62 if needed. The fan 64 may be electronicallycontrolled. The temperature sensor 68 may be located just upstream ofthe fuel-cell stack 22 to measure the temperature of the coolant at theinlet 74. The temperature sensor 70 may be located just downstream ofthe fuel cell 22 to measure temperature of the coolant at the outlet 76.

The first and second coolant circuits 32 and 34 may also be used to coolor heat the compressed air supplied to the fuel cells 20, 22. Anintercooler 80 is connected to the conduit 44 and a compressed-air line82. The intercooler 80 is configured to exchange heat between thecompressed air and the coolant. An intercooler 84 is connected to theconduit 72 and a compressed air line 86. The intercooler 84 isconfigured to exchange heat between the compressed air and the coolant.

A valve arrangement 90 selectively connects the first circuit 32 and thesecond circuit 34 in fluid communication. A valve arrangement is acollection of one or more valves configured to modify fluid flow betweenone or more inlet ports and one or more outlet ports. A valvearrangement may include multiple valves housed in a common body ormultiple discrete valves acting in unison with each other. The valvearrangement may be passive or electrically controlled. Example valvearrangements include two-way valves, three-way valves, four-way valves,check valves, an isolation valve, and the like. When the valvearrangement 90 is closed, the first and second circuits 32, 34 areseparate and the coolant in those circuits does not mix. When the valvearrangement 90 is open, the first and second circuits 32, 34 are influid communication with each other and the coolant mixes. The valvingarrangement 90 may be electronically controlled by controller (such ascontroller 91) to connect the first and second circuits 32, 34 in fluidcommunication and to isolate the first and second circuits 32, 34. Usedherein “isolate” means to sever or prevent fluid communication. Thecontroller 91 may be programmed to actuate the valve arrangement 90between the closed position and a plurality of different open positions(sometimes called mixing positions) that proportion the amount ofcoolant mixing between first and second circuits.

Referring to FIG. 3, according to one or more embodiments, the valvearrangement 90 is an isolation valve assembly 92 having a housing 94with ports and valving. For example, the housing 94 may include a firstinlet port 96, a second inlet port 98, a first outlet port 100, and asecond outlet port 102. The first inlet port 96 and the first outletport 100 are connected to the conduit 44 of the first circuit 32, andthe second inlet port 98 and the second outlet port 102 are connected tothe conduit 72 of the second circuit 34. Valving 104 is disposed in thehousing 94 and controls the flow of fluid between the inlet and outletports.

In the illustrated embodiment, the valving 104 includes a first valve106 and a second valve 108. The first valve 106 is associated with thefirst inlet port 96 and controls the flow of fluid from the inlet port96 between the first and second outlet ports 100 and 102. A first fluidpath 110 connects between the inlet port 96 and the outlet port 100 andthe second fluid path 112 connects between the inlet port 96 and theoutlet port 102. The valve 106 is configured to proportion coolantbetween the paths 110 and 112 depending upon the position of the valve106. The valve 106 includes a first position in which all of the fluidflows from the inlet port 96 to the outlet port 100, a second positionin which all the fluid flows from the inlet port 96 to the outlet port102, and intermediate positions in which coolant flows through both thefirst path 110 and the second path 112. The second valve 108 isassociated with the second inlet port 98 and controls the flow of fluidfrom the inlet port 98 between the first and second outlet ports 100 and102. A third fluid path 114 connects between the inlet port 98 and theoutlet port 102 and a fourth fluid path 116 connects between the inletport 98 and the outlet port 100. The valve 108 is configured toproportion coolant between the paths 114 and 116 depending upon theposition of the valve. The valve 108 includes a first position in whichall of the fluid flows from the inlet port 98 to the outlet port 102, asecond position in which all the fluid flows from the inlet port 98 tothe outlet port 100, and intermediate positions in which coolant flowsthrough both the third path 114 and the fourth path 116.

When the isolation valve assembly 92 is in the closed position, thevalves 106 and 108 are in their first positions so that fluid flows onlythrough the first path 110 and the third path 114. When the isolationvalve assembly 92 is in the fully open position, the valves 106 and 108are in their second positions so that fluid flows only through thesecond path 112 and the fourth path 116. When the isolation valveassembly 92 is in one or more partially open positions, the valves 106108 are in one of their intermediate positions so that the fluid flowsthrough all four paths 110, 112, 114, and 116. The valves 106 and 108may be synchronized so that movement of the valve 106 results in aproportional movement of the valve 108 and vice versa. For example, ifthe valve 106 is actuated so that 80 percent of the coolant is routed tothe first outlet 100 and 20 percent of the coolant is routed to theoutlet 102, the valve 108 is actuated so that 80 percent of the coolantis routed to the second outlet 102 and 20 percent of the coolant isrouted to the first outlet 100.

Referring to FIG. 4, according to an alternative embodiment, the valvearrangement 90 is collection of two three-way valves 120, 130 andassociated conduit. The first three-way valve 120 includes an inlet port122 connected to the conduit 44 an outlet port 124 connected to theconduit 44 and an outlet port 126 connected to the conduit 72 by aconduit 128 that connects downstream of the valve 130. The three-wayvalve 120 may be configured to route all the coolant to the outlet 124,all of the coolant the outlet 126, or proportion coolant between bothoutlets 124, 126. The second three-way valve 130 includes an inlet port132 connected to the conduit 72 an outlet port 134 connected to theconduit 72 and an outlet port 136 connected to the conduit 44 by aconduit 138 that connects downstream of the valve 120. The three-wayvalve 130 may be configured to route all the coolant to the outlet 134,all of the coolant the outlet 136, or proportion coolant between bothoutlets 134, 136. FIGS. 3 and 4 are just two examples of the valvearrangement 90 and are not limiting.

Referring back to FIG. 2, the thermal management system 30 also includesa third coolant circuit 140 associated with the traction battery 14 andthe heating ventilation and air-conditioning system (HVAC) system. Thethird circuit 140 may include a pump 142, a heater 144, heat exchanger146, a heater core 147, a valve 148 (e.g., a four-way valve), an ON-OFFvalve 150, an ON-OFF valve 152, and conduit 154 configured to circulatecoolant between these various components. (The valves 150 and 152 may bereferred to as a valve arrangement). The heater 144 may be an electricheater configured to convert electricity into thermal energy that heatsthe coolant. For example, the heater 144 may be a positive temperaturecoefficient (PTC) heater. The heater core 147 may be disposed within theHVAC unit and transfers heat from the coolant of the third circuit 140to air bound for the passenger cabin. The heat exchanger 146 may be aliquid-to-liquid heat exchanger. The heat exchanger 146 is connected tothe conduit 154 and to conduit 156 of the battery cooling circuit 157.The heat exchanger 146 is configured to transfer thermal energy betweenthe conduit 154 and the conduit 156 without mixing the fluids.

Four conduits connect the third circuit 140 to the first and secondcircuits 32, 34. A conduit 158 connects the first circuit 32 to thethird circuit 140 at the valve 148, and a conduit 164 connects thesecond circuit 34 to the third circuit 140 at the valve 148. Additionalconduit 160 and 162 connect to the circuits together as well. Theconduits may act as return conduit to the first and second circuits. Thevalves 148, 150, and 152 open and close in cooperation to fluidlyconnect the third circuit 140 to the first circuit 32 and/or the secondcircuit 34, and to isolate the third circuit. The circuit 140 can beisolated from circuits 32 and 34 via the four-way valve by blockingconduit 158 and conduit 164 and allowing flow through the port 170.

According to one embodiment, the valve 148 is a four-way valve thatincludes a first port 166 connect to the conduit 158, a second port 168supplying coolant to the pump 142, third port 170, and fourth port 172.The ports 166, 170, and 172 may be inlet ports and the port 168 may bean outlet port. The inlet ports may be individually controlled betweenopened, closed, and/or throttled positions (in some embodiments, onlyports 166 and 172 are proportional). The four-way valve 148 may beelectronically controlled by the controller 91. The valves 150 and 152,which may also be controlled by the controller 91, regulate the flow offluid through the conduits 160 and 162, respectively.

As discussed above, it is difficult to start a fuel cell when thetemperature is below a certain threshold. The fuel cell may be warmedabove a starting temperature (temperature sufficient for reliablestarting) using a heater, e.g., heater 144, powered by the battery 14.Heating coolant using electricity is inefficient and it is advantageousto utilize the heater 144 as little as possible so that more of thestored energy in the battery 14 can be used for propulsion. According toone embodiment, the heater 144 is used to heat only one of the fuel-cellstacks 20, 22 in a preconditioning step. In this example, the stack 20is heated. Once the fuel-cell stack 20 reaches the starting temperature,that fuel-cell stack 20 is started and the heater 144 is turned OFF(unless cabin or battery heating requires use of the heater 144). Thefuel-cell stack 20 continues to run heating itself up. Once the coolantof the first circuit reaches a threshold temperature, waste heat fromthe fuel-cell stack 20 is used to precondition the second fuel-cellstack 22. The second fuel-cell stack 22 can be started once it reachesits starting temperature. This sequence of events will be described inmore detail below.

The thermal management system 30 includes a plurality of differentmodes. Representative examples of these modes will be described in FIGS.5 through 9 and the associated text. (Solid lines indicate activecoolant flow, dashed lines indicate inactivity). FIG. 5 illustrates apreconditioning mode for warming the first fuel-cell stack 20 prior tostarting. The preconditioning mode 200 warms the fuel-cell stack 20 to atemperature sufficient for reliable starting. In this mode, only thefirst stack 20 is being heated by the heater 144. In the preconditioningmode 200, the first circuit 32 and the third circuit 140 are active andare connected in fluid communication with each other by actuating valves148 and 150 accordingly. The second circuit 34 is deactivated andisolated from the other circuits by actuating valves 148 and 152accordingly. The first and third circuits 32, 140 are connected in fluidcommunication by opening the valve 150 and actuating the valve 148 sothat the inlet 166 is connected in fluid communication with the outlet168 and the other and inlets of valve 148 are OFF. Closing the inlet 172and actuating the valve 152 to the closed position isolates the secondcircuit 34 from the third circuit 140. The valve arrangement 90 isclosed to isolate the first and second circuits 32, 34, e.g., the valve106 is actuated so that all of the coolant flowing from the inlet 96flows through path 110 to the outlet 100. In mode 200, the pumps 36, 142are ON, the PCT heater 144 is ON, and the valve 42 is actuated to bypassthe radiator 38.

Starting at the pump 142, coolant circulates through the heater 144where the coolant is warmed and subsequently flows into the firstcircuit 32 through the open valve 150. The heated coolant then passesthrough the pump 36 and subsequently through the fuel-cell stack 20 andthe intercooler 80. In some embodiment, the intercooler 80 may bebypassed to reduce heating time for the fuel cell. The heated coolantwarms the stack 20 and the discharged coolant re-circulates to the pump142 through the four-way valve 148. The HVAC system and the batterycoolant loop may be deactivated so that all of the heat from the heater144 is used to warm the fuel-cell stack 20.

The vehicle 10 may include a preconditioning mode for the second stack22. This mode is a mirror of the mode 200 except that the second circuit34 is active and connected to the third circuit 140 and the firstcircuit 32 is OFF.

FIG. 6 illustrates a mode 202, which may be referred to as warm-up mode,used post cold start of the fuel-cell stack 20. The mode 202 may beutilized following mode 200. In this mode, coolant in the first circuit32 is isolated while the fuel cell 20 heats up using self-generatedwaste heat. The first circuit 32 remains isolated from the secondcircuit 34 and is now isolated from the third circuit 140 by actuatingthe four-way valve 148 to close the inlets 166 and 172. In mode 202, thethird circuit 140 may be activated or deactivated depending upon theneeds of the battery 14 and the cabin. In FIG. 6, the third circuit 140is shown activated (pump and heater ON) to heat the battery 14 and thepassenger cabin. A similar mode may be used for the second fuel-cellstack 22 if it is started first.

FIG. 7 illustrates a fuel-cell-to-fuel-cell warmup mode 204 where wasteheat from the fuel-cell stack 20 is used to preheat the fuel-cell stack22 to a startup temperature. In mode 204, the first circuit 32 and thesecond circuit 34 are connected in fluid communication via the valvearrangement 90 and the third circuit 140 is isolated. The third circuit140 may be activated or deactivated depending upon the needs of thebattery and the cabin. In FIG. 7, the third circuit 140 is activated(pump and heater ON) to heat the battery 14 and the passenger cabin.

The valves 150 and 152 are actuated to the closed position to preventthe flow of fluid from the third circuit 140 to the first and secondcircuits 32, 34. The four-way valve 148 is actuated so that the ports166 and 172 are OFF and the ports 170 and 168 are open so that fluidcirculates only through the third circuit 140. The pumps 36 and 60 areenergized to circulate coolant and the valves 42 and 66 are actuated tobypass the radiators 38 and 62. The valve arrangement 90 is in one ofthe open (mixing) positions to connect the first circuit 32 and thesecond circuit 34 in fluid communication. During operation, heatedcoolant exiting the stack 20 is circulated through the valve arrangement90 so that at least some of the fluid flows to the second circuit 34 andsubsequently through the fuel-cell stack 22 to heat the stack. The valvearrangement 90 is controlled so that the temperature of the coolant inthe first circuit 32 is maintained above a desired temperature. Ingeneral, the valve arrangement 90 is actuated to gradually increase themixing of the circuits 32, 34 as the second stack 22 warms. For example,the controller 91 will control the inlet temperature of the running fuelcell (e.g. first stack 20) within a desired range, while monitoring theoutlet temperature of the fuel cell 20 such that the temperaturedifference between the inlet and the outlet of the stack 20 (e.g.,sensors 46, 48) is also within a desired range. In the second circuit34, the controller 91 monitors one or more of the temperature sensors todetermine if the stack 22 is ready to start.

In the vehicle 10, the second stack 22 may be used to warm the firststack 20 using a mode similar to the mode 204. This may be used, forexample, if the second stack 22 is preconditioned using the heater 144as described in the above non-illustrated mode.

Referring to FIG. 8, once both stacks 20, 22 reach an operatingtemperature range, the vehicle may enter into a cabin-and-batteryheating mode 206. In mode 206, the third circuit 140 is in fluidcommunication with the first and second circuits 32, 34, but the valvearrangement 90 is closed to isolate the first and second circuits 32, 34from each other. (In other embodiments or conditions, the valvearrangement 90 may be open to connect the first and second circuits 32,34 in fluid communication.) The heater 144 is OFF and heat from thefuel-cell stacks 20 and 22 are used to heat the battery 14 and thecabin. Heated coolant discharge from the first stack 20 is circulated tothe inlet port 166 of the four-way valve 148 and heated coolantdischarge from the second stack 22 is circulated to the inlet port 172of the four-way valve. The four-way valve is actuated to route bothinlet ports 166 and 172 to the outlet port 168 connected to the pump142. The pump 142 circulates the heated coolant through the heatexchanger 146 and the heater core 147 to heat the battery 14 and thecabin. This reduces the temperature of the coolant which is thencirculated back to the first and second circuits 32 and 34. In FIG. 8,the radiators 38 and 62 are being bypassed, however, if the heater core147 and the heat exchanger 146 are insufficient to cool the coolant, oneor more of these radiators may be used as needed to maintain theirrespective fuel-cell stack within the desired temperature range.

In another operating mode similar to mode 204, the third circuit 140 maybe deenergized if neither the battery nor the cabin are requesting heat.Here, the valves 150, 152 may be closed and the four-way valve 148 maybe actuated to close inlets 166, 172.

The vehicle 10 may only operate on one of the fuel cells at a timedepending upon the operating needs of the vehicle. FIG. 9 illustrates anexample where the first stack 20 is ON and the second stack 22 is OFF.In this mode, 208, the valve 152 and the valve arrangement 90 areactuated to the closed positions to prevent the flow of fluid to andfrom the second circuit 34. In mode 208, the fuel-cell stack 20 is beingused to heat the cabin and the battery and the heater 144 is OFF. Thevalve 150 is open and the four-way valve 148 is actuated so that coolantflows from the port 166 to the port 168 as described above in FIG. 8.The valve 42 is operated to maintain the coolant temperature within adesired range. The vehicle 10 may also be operated with second stack 22only and have another mode that is the mirror of mode 208.

FIG. 10 is a control diagram for the thermal management system 30 andshows a non-exhaustive list of component in electric communication withthe controller 91 that controls operation of at least the system 30.While illustrated as one controller, the controller 91 may be part of alarger control system and may be controlled by various other controllersthroughout the vehicle 10, such as a vehicle system controller (VSC).The VSC may control the various vehicle components such the thermalmanagement system 30, the fuel-cell stacks, and many others. It shouldtherefore be understood that the controller 91 and one or more othercontrollers of the VSC can collectively be referred to as a “controller”that controls various actuators in response to signals from varioussensors to control functions of the thermal management system 30 and thefuel-cell stacks. Controller 91 may include a microprocessor or centralprocessing unit (CPU) in communication with various types ofcomputer-readable storage devices or media. Computer-readable storagedevices or media may include volatile and nonvolatile storage inread-only memory (ROM), random-access memory (RAM), and keep-alivememory (KAM), for example. KAM is a persistent or non-volatile memorythat may be used to store various operating variables while the CPU ispowered down. Computer-readable storage devices or media may beimplemented using any of a number of known memory devices such as PROMs(programmable read-only memory), EPROMs (electrically PROM), EEPROMs(electrically erasable PROM), flash memory, or any other electric,magnetic, optical, or combination memory devices capable of storingdata, some of which represent executable instructions, used by thecontroller in controlling the vehicle.

The controller communicates with various vehicle sensors and actuatorsvia an input/output (I/O) interface that may be implemented as a singleintegrated interface that provides various raw data or signalconditioning, processing, and/or conversion, short-circuit protection,and the like. Alternatively, one or more dedicated hardware or firmwarechips may be used to condition and process particular signals beforebeing supplied to the CPU. Although not explicitly illustrated, those ofordinary skill in the art will recognize various functions or componentsthat may be controlled by controller 91 within each of the subsystemsidentified above.

The controller 91 is in electric communication with the temperaturesensors 46, 48, 68, and 70 as well as any additional temperature sensorsor pressure sensors (not illustrated) if provided. Each temperaturesensor 46 is configured to output a signal to the controller 91indicative of a sensed temperature of the coolant or other measuredcomponent. The controller 91 includes logic for interpreting the signalsand commanding the various actuators accordingly. The controller 91 mayoutput signals to the pumps 36, 60, 142 including an activation state(ON or OFF) and a speed setting. The valve arrangement 90 and the valves150, 152 are also in electric communication with the controller 91. Thecontroller 91 is configured to command actuation of these valves. One ormore the valves may include an associated actuator that moves the valvebetween various positions to control flow of coolant through the valve.The controller 91 may also control operation of the radiator fans 40 and64, and any other electronic valves.

Control logic or functions performed by controller 91 may be representedby flow charts or similar diagrams in one or more figures. These figuresprovide representative control strategies and/or logic that may beimplemented using one or more processing strategies such asevent-driven, interrupt-driven, multi-tasking, multi-threading, and thelike. As such, various steps or functions illustrated may be performedin the sequence illustrated, in parallel, or in some cases omitted.Although not always explicitly illustrated, one of ordinary skill in theart will recognize that one or more of the illustrated steps orfunctions may be repeatedly performed depending upon the particularprocessing strategy being used. Similarly, the order of processing isnot necessarily required to achieve the features and advantagesdescribed herein, but is provided for ease of illustration anddescription. The control logic may be implemented primarily in softwareexecuted by a microprocessor-based vehicle, engine, and/or powertraincontroller, such as controller 91. Of course, the control logic may beimplemented in software, hardware, or a combination of software andhardware in one or more controllers depending upon the particularapplication. When implemented in software, the control logic may beprovided in one or more computer-readable storage devices or mediahaving stored data representing code or instructions executed by acomputer to control the vehicle or its subsystems. The computer-readablestorage devices or media may include one or more of a number of knownphysical devices which utilize electric, magnetic, and/or opticalstorage to keep executable instructions and associated calibrationinformation, operating variables, and the like.

FIG. 11 is a flowchart 300 of an algorithm for controlling the thermalmanagement system 30 during cold starting of the fuel-cell stacks. Theflowchart 300 illustrates an embodiment in which the first fuel-cellstack 20 is started first, but this may be switched in otherembodiments. The controls began at operation 302 where the driver orvehicle has requested starting of the fuel-cell stack 20. At operation301, the controller determines if the temperature of the stack 20 isless than a first threshold. The controller may determine thetemperature based on signals from one or more of the temperature sensorsof the thermal management system 30. The first threshold may be calledthe cold-start threshold and may have a value of −25 degrees C. forexample. If yes at operation 301, the fuel-cell stack 20 is too cold tobe reliably started and control passes to operation 304 to preconditionthe stack 20 until the temperature exceeds the first threshold.Operation 304 may include commanding the thermal management system tomode 200 shown in FIG. 5. The controller may command mode 200 bycommanding the valve arrangement 90 to the closed (isolation) position,commanding the heater ON, commanding the valve 150 to open, commandingthe valve 152 closed, commanding the four-way valve 148 to only routecoolant from the inlet 166 to the outlet 168, and energizing the pumps36 and 142.

The thermal management system 30 remains in the mode 200 until thefuel-cell stack 20 exceeds the cold-start threshold. Once the stack 20exceeds the cold-start threshold (determined at operation 306), controlpasses to operation 308 and the fuel-cell stack 20 is started. Controlpasses to operation 310 after the fuel-cell stack 20 is started. Atoperation 310, the thermal management system 30 is switched to mode 202.The controller may switch from mode 200 to mode 202 by closing the valve150 and actuating the four-way valve 148. The thermal management system30 remains in mode 202 until the fuel-cell stack 20 exceeds a secondthreshold. The second threshold may be called a warm-up threshold andmay have a value of 50 degrees C. for example.

If the fuel-cell stack 20 exceeds the second threshold at operation 311,control passes to operation 312 the controller determines if there is arequest to start the second fuel-cell stack 22. If yes, control passesto operation 314 and the controller determines if the second stack 22 isless than the cold-start threshold. If it is, control passes tooperation 316 and the controller commands the thermal management system30 to mode 204. The controller may switch from mode 202 to mode 204 byopening the valve arrangement 90 to an initial mixing position thatroutes a small amount of coolant through the paths 112 and 116 to beginmixing the first and second circuits 32, 34 and warming the second stack22 with waste heat of the first stack 20. The controller may beprogrammed to gradually open the valve arrangement 90 to heat the secondstack 22 while preventing the coolant temperature of the first circuit32 from falling below a desired operating range. The pumps 36, 60 may becommanded to run at the same speed.

At operation 318, the controller determines if the temperature of thefuel-cell stack 22 exceeds the first threshold. If no, the thermalmanagement system 30 remains in mode 204 until the second fuel-cellstack 22 is heated to the first threshold. The fuel-cell stack 22 isstarted at operation 320 once the first threshold temperature isachieved. The controller commands the thermal management system 30 tothe mode 206 at operation 322. The controller may switch the thermalmanagement system 30 from mode 204 to mode 206 by closing the valvearrangement 90. In mode 206, the first circuit 32 and/or the secondcircuit 34 may be used to heat the cabin, the traction battery 14, orboth. In the illustrated mode 206, both circuits 32, 34 are used, inwhich case, the valves 150, 152 are open and the four-way valve 148 isactuated to route coolant from the inlet 166 to the outlet 168 to placethe first and third circuits 32, 140 in fluid communication and to routecoolant from the inlet 172 to the outlet 168 to place the second andthird circuits 34, 140 in fluid communication. Using waste heat of thefuel-cell stacks 20, 22 to heat the battery 14 and the cabin allows theheater 144 to be deenergized allowing for more of the electricitygenerated by the fuel cells for use for propulsion.

If no at operation 312, control passes to operation 324 and thecontroller determines if the cabin or battery are requesting heat. Ifno, control passes to other control logic which will not be discussedhere. If yes, control passes operation 326 and the controller commandsthe thermal management system 30 to the mode 208. In mode 208, thesecond circuit 34 is deenergized since the fuel-cell stack 22 is OFF andthe first and third circuits 32, 140 are connected in fluidcommunication. Waste heat from the fuel-cell stack 20 is used to heatthe passenger cabin via the heater core 147 and to heat the battery 14via the heat exchanger 146. The heater 144 may be OFF if the waste heatof the fuel-cell stack 20 is sufficient to provide the requested heatingof the battery and the cabin. Otherwise the heater 144 may be activatedto provide additional heat.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes caninclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

What is claimed is:
 1. A vehicle comprising: first and second fuel-cellstacks; and a thermal management system including: a first coolantcircuit including conduit arranged to circulate coolant through thefirst fuel-cell stack, a first pump, and a first radiator, a secondcoolant circuit including conduit arranged to circulate coolant throughthe second fuel-cell stack, a second pump, and a second radiator, athird coolant circuit including conduit arranged to circulate coolantthrough a heater, a third pump, and a heater core, a first valvearrangement configured to selectively connect the third circuit to thefirst circuit and configured to selectively connect the third circuit tothe second circuit, and a second valve arrangement configure toproportion a flow of coolant between the first and second coolantcircuits.
 2. The vehicle of claim 1 further comprising a batteryassembly, wherein the thermal management system further includes: a heatexchanger connected to the conduit of the third circuit, and a fourthcoolant circuit including conduit arranged to circulate coolant throughthe battery assembly and the heat exchanger.
 3. The vehicle of claim 1,wherein the thermal management system further includes: a third valvearrangement including a first port connected to the conduit of the thirdcircuit and a second port connected to the conduit of the first circuit,and a fourth valve arrangement including a first port connected to theconduit of the third circuit and a second port connected to the conduitof the second circuit.
 4. The vehicle of claim 1, wherein the secondvalve arrangement includes a first inlet port connected to the conduitof the first circuit, a first outlet port connected to the conduit ofthe first circuit, a second inlet port connected to the conduit of thesecond circuit, a second outlet port connected to the conduit of thesecond circuit, and valving having (i) an isolation position in whichthe first inlet port and the second outlet port are not in fluidcommunication and the second inlet port and the first outlet port arenot in fluid communication so that the first and second circuits areisolated and (ii) at least one mixing position in which the first inletport and the second outlet port are in fluid communication and thesecond inlet port and the first outlet port are in fluid communicationso that the first and second circuits are in fluid communication.
 5. Thevehicle of claim 1, wherein the thermal management system has (i) afirst mode in which the first valve arrangement connects only the firstand third circuits in fluid communication, the heater is ON, and thesecond valve arrangement isolates the first and second circuits, (ii) asecond mode in which the first valve arrangement isolates the thirdcircuit from the first and second circuits and the second valvearrangement isolates the first and second circuits, and (iii) a thirdmode in which the first valve arrangement isolates the third circuitfrom the first and second circuits and the second valve arrangementconnects the first and second circuits in fluid communication.
 6. Thevehicle of claim 1, wherein the first valve arrangement is a four-wayvalve having first and second ports connected to the conduit of thethird circuit, a third port connected to conduit of the first circuit,and a fourth port connected to conduit of the second circuit.
 7. Avehicle comprising: first and second fuel-cell stacks; a first coolantcircuit including conduit arranged to circulate coolant through thefirst fuel-cell stack; a second coolant circuit including conduitarranged to circulate coolant through the second fuel-cell stack; aheater in fluid communication with at least the first coolant circuit; avalve arrangement configured to proportion a flow of coolant between thefirst and second coolant circuits, the valve arrangement including anisolation position in which the first and second circuits are not influid communication and at least one mixing position in which the firstand second circuits are in fluid communication; and a controllerprogrammed to: in response to coolant of the first circuit being lessthan a first threshold, command the isolation valve assembly to theisolation position and command the heater ON, and in response to coolantof the first circuit exceeding a second threshold, command the isolationvalve assembly to the at least one mixing position and command theheater OFF.
 8. The vehicle of claim 7, wherein the controller is furtherprogrammed to start the first fuel cell in response to the coolant ofthe first circuit exceeding the first threshold.
 9. The vehicle of claim7, wherein the controller is further programmed to, in response tocoolant of the second circuit exceeding the second threshold, commandstarting of the second fuel cell and command the isolation valveassembly to the isolation position.
 10. The vehicle of claim 7 furthercomprising: a third coolant circuit including conduct arranged tocirculate the coolant through the heater; and a second valve arrangementconfigured to selectively connect the third circuit to the first circuitand configured to selectively connect the third circuit to the secondcircuit, wherein the controller is further programmed to actuate thesecond valve arrangement to connect the first and third circuits inresponse to the coolant of the first circuit being less than the firstthreshold.
 11. The vehicle of claim 10, wherein the controller isfurther programmed to actuate the second valve arrangement to isolatethe first and third circuits in response to the coolant of the firstcircuit exceeding the first threshold.
 12. A vehicle comprising: firstand second fuel-cell stacks; a first coolant circuit including conduitarranged to circulate coolant through the first fuel-cell stack; asecond coolant circuit including conduit arranged to circulate coolantthrough the second fuel-cell stack; a heater in fluid communication withat least the first coolant circuit; a valve arrangement configured toproportion a flow of coolant between the first and second coolantcircuits, the valve arrangement including an isolation position in whichthe first and second circuits are not in fluid communication and atleast one mixing position in which the first and second circuits are influid communication.
 13. The vehicle of claim 12 further comprising: athird coolant circuit including conduct arranged to circulate thecoolant through the heater; and a second valve arrangement configured toselectively connect the third circuit to the first circuit andconfigured to selectively connect the third circuit to the secondcircuit, wherein the controller is further programmed to actuate thesecond valve arrangement to connect the first and third circuits inresponse to the coolant of the first circuit being less than the firstthreshold.
 14. The vehicle of claim 12 further comprising a batteryassembly.